Optical transmitter, optical receiver, and optically coupled insulating device

ABSTRACT

According to one embodiment, an optically coupled insulating device includes an optical transmitter and an optical receiver. The optical transmitter includes an analog-to-digital converter, an encoder, a transmitting controller, and an electrooptical transducer. The encoder is configured to generate a transmitting signal by superimposing an output of the analog-to-digital converter onto a signal based on a clock signal. The transmitting signal is encoded to have an average duty ratio of more than zero and less than one. The transmitting controller is configured to output one of the transmitting signal and the output of the analog-to-digital converter depending on an input level of the analog signal. The electrooptical transducer is configured to convert an output of the transmitting controller into an optical signal. The optical receiver includes an optoelectrical transducer, a decoder, and a receiving controller.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-018769, filed on Jan. 29, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an optical transmitter,an optical receiver, and an optically coupled insulating device.

BACKGROUND

Insulating circuits having high noise immunity such as insulatingamplifiers are used in environments such as electric vehicles andapparatuses for factory automation where high power electrical devicescoexist with highly sensitive electronic devices. Optically coupledinsulating devices such as photocouplers that optically transmit signalshave excellent noise immunity because complete electrical insulation isprovided between inputs and outputs.

In optically coupled insulating devices, analog signal data can betransmitted with high precision to circumvent nonlinearity ofphotocouplers and the like by converting detected analog signals intodigital signals before being transmitted optically. Further, opticallycoupled insulating devices have been proposed in which encoded digitalsignals and clock signals are transmitted by photocouplers and the likein order to reduce the number of transmission lines (for example, referto JP-A 2006-303663 (Kokai)).

With regard to optical connection of optically coupled insulatingdevices, verification of normal transmission is necessary during thespecified guarantee period even if temperature change and agingdegradation of a light emitting element such as a light emitting diode(LED) occur. Simplified testing methods are preferable in a producttesting process for the optically coupled insulating devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of an opticallycoupled insulating device including an optical transmitter and anoptical receiver according to an embodiment;

FIG. 2 is a circuit diagram illustrating the configuration of a switchcontroller of the optically coupled insulating device illustrated inFIG. 1;

FIG. 3 is a block diagram illustrating the configuration of anotheroptical transmitter according to the embodiment;

FIG. 4 is a block diagram illustrating the configuration of stillanother optical transmitter according to the embodiment;

FIG. 5 is a truth table of inputs and outputs of an encoder;

FIG. 6 is a schematic view of a truth table of inputs and outputs andpulse waveforms; and

FIG. 7 is a schematic view of another truth table of inputs and outputsand pulse waveforms.

DETAILED DESCRIPTION

In general, according to one embodiment, an optically coupled insulatingdevice includes an optical transmitter and an optical receiver. Theoptical transmitter includes an analog-to-digital converter, an encoder,a transmitting controller, and an electrooptical transducer. Theanalog-to-digital converter is configured to convert an analog signalinto a digital signal. The encoder is configured to generate atransmitting signal by superimposing an output of the analog-to-digitalconverter onto a signal based on a clock signal. The transmitting signalis encoded to have an average duty ratio of more than zero and less thanone. The transmitting controller is configured to output one of thetransmitting signal and the output of the analog-to-digital converterdepending on an input level of the analog signal. The electroopticaltransducer is configured to convert an output of the transmittingcontroller into an optical signal. The optical receiver includes anoptoelectrical transducer, a decoder, and a receiving controller. Theoptoelectrical transducer is configured to receive the optical signaland to convert the optical signal into an electrical signal. The decoderis configured to decode an output of the optoelectrical transducer andto output a regenerated digital signal and a regenerated clock signal.The receiving controller is configured to output one of an output of thedecoder and the output of the optoelectrical transducer depending on anaverage duty ratio of the output of the optoelectrical transducer.

Embodiments will now be described in detail with reference to thedrawings. In the specification and the drawings of the application,components similar to those described in regard to a drawingthereinabove are marked with like reference numerals, and a detaileddescription is omitted as appropriate. Furthermore, L denotes a logicalvalue of a low-level and H denotes a logical value of a high-level.

FIG. 1 is a block diagram illustrating the configuration of an opticallycoupled insulating device including an optical transmitter and anoptical receiver according to an embodiment.

The optically coupled insulating device 1 includes the opticaltransmitter 2 and the optical receiver 3 as illustrated in FIG. 1

The optical transmitter 2 will now be described.

The optical transmitter 2 includes an analog-to-digital converter 4, anencoder 5, a driver 6, an electrooptical transducer 7, and atransmission controller 8. The analog-to-digital converter 4, theencoder 5, the driver 6, and the transmission controller 8 are formed inthe same semiconductor substrate to provide a one-chip structure.

The analog-to-digital converter 4 samples an analog signal input with asampling period determined by a clock signal and converts the analogsignal into a digital signal. The analog-to-digital converter 4digitizes the analog signal input with desired precision and outputs as,for example, an n-bit digital signal (n being a positive integer). Asthe analog-to-digital converter 4, for example, a delta-sigmaanalog-to-digital converter (ΔΣADC) may be used. In this case, a digitalsignal to be output is low bit, e.g., one-bit data, and theconfiguration of the encoder 5 is simplified.

The output of the analog-to-digital converter 4 is synchronized with theclock signal and is input to the encoder 5.

The encoder 5 encodes the digital signal of the output of theanalog-to-digital converter 4 and the clock signal into a transmittingsignal, e.g., a PWM signal, of one transmission line. In other words,the encoder 5 generates the transmitting signal in which the output ofthe analog-to-digital converter 4 is superimposed onto the clock signal.The transmitting signal is encoded to have an average duty ratio D ofmore than zero and less than one. The not-illustrated clock signal isgenerated by a clock generator (not illustrated) formed the samesemiconductor substrate or an external clock generator (not illustrated)and applied to the analog-to-digital converter 4. The clock signaldefines a sampling period.

The output of the analog-to-digital converter 4 can be any value fromconsecutive Ls to consecutive Hs. However, the transmitting signal ofthe output of the encoder 5 is decoded to a regenerated digital signaland a regenerated clock signal by the optical receiver 3, as describedbelow. Therefore, the transmitting signal is encoded by the encoder 5 sothat the optical receiver 3 can regenerate a clock signal from thetransmitting signal.

Therefore, the average duty ratio D of the transmitting signal is withinspecified values between zero and one, which is determined by anencoding scheme, and neither zero nor one. Further, in order to easilyregenerate the clock signal, the transmitting signal is encoded so thatperiods of consecutive Ls and consecutive Hs (run length) are not toolong with respect to the period of the clock signal. Therefore, thespecified values of the average duty ratio D of the transmitting signalare proximal to middle values between zero and one, and proximal toneither zero nor one.

The electrooptical transducer 7 converts the output of the transmissioncontroller 8 into an optical signal and outputs the optical signal. Theelectrooptical transducer 7 consists of, for example, a light emittingelement such as a LED.

The electrooptical transducer 7 converts the transmitting signal outputfrom the encoder 5 or the analog signal input via the transmissioncontroller 8 into an optical signal, as described below.

The driver 6 receives the transmitting signal of the output of theencoder 5 and outputs the transmitting signal to the transmissioncontroller 8. In FIG. 1, the configuration is illustrated in which thedriver 6 is connected to an LED as the electrooptical transducer 7 anddraws in currents from the cathode side of the LED. However, the driver6 may pass currents into, for example, the anode side of the LED.Alternatively, the driver 6 may be included in the encoder 5.

The transmission controller 8 includes a switching element 9 and aswitch controller 10. The switching element 9 outputs one selected fromthe output of the driver 6 and the analog signal input. The switchcontroller 10 controls the switching element 9 by determining whether anormal operating state or a testing state for an optical connectiondepending on an input level V_(IN) of the analog signal.

The switch controller 10 controls the switching element 9 to the normaloperating state or the testing state for an optical connection dependingon the input level V_(IN) of the analog signal with respect to, forexample, the maximum value (full scale input) V_(IN) _(—) _(MAX) of theinput level of the analog-to-digital converter 4. In the normaloperating state, the input level of the analog signal is limited inV_(IN)<V_(IN) _(—) _(MAX). In this case, the switch controller 10controls the switching element 9 as the normal operating state to outputthe output of the encoder 5 via the driver 6. In the testing state forthe optical connection, the input level of the analog signal is set inV_(IN)>V_(IN) _(—) _(MAX). In this case, the switch controller 10controls the switching element 9 as the testing state for the opticalconnection to output the analog signal input.

Therefore, the transmission controller 8 outputs the output of theencoder 5 to the electrooptical transducer 7 via the driver 6 in thenormal operating state. The transmission controller 8 outputs the analogsignal input to the electrooptical transducer 7 in the testing state forthe optical connection.

The optical transmitter 2 is controlled in the normal operating state orthe testing state for the optical connection by the transmissioncontroller 8 depending on the input level V_(IN) of the analog signal.

When the withstand voltage of the optical transmitter 2 is V_(MAX), therelation of V_(IN) _(—) _(MAX)<V_(MAX) is satisfied. For example, in thecase of a delta-sigma analog-to-digital converter (ΔΣADC), the maximumvalue V_(IN) _(—) _(MAX) of the input level is equal to an internalreference voltage V_(REF). However, the reference voltage V_(REF) isgenerated by using, for example, a so-called bandgap-voltage generatorand an amplifier. Therefore, with respect to a supply voltage V_(DD),the relation of V_(REF)<V_(DD) is satisfied and the relation of V_(IN)_(—) _(MAX)<V_(MAX) is guaranteed. In other words, in the normaloperating state, the relation of V_(IN)<V_(IN) _(—)_(MAX)=V_(REF)<V_(MAX) is satisfied. In the testing state for theoptical connection, the relation of V_(IN) _(—)_(MAX)=V_(REF)<V_(IN)<V_(MAX) is satisfied.

Therefore, in the normal operating state, the analog signal input issampled by the analog-to-digital converter 4 with the sampling periodand converted into the n-bit digital signal. For example, in the case ofthe ΔΣADC, the analog signal is converted into one-bit digital signal.

Then, the digital signal and the clock signal are encoded into onetransmitting signal by the encoder 5. The output of the encoder 5, i.e.,the transmitting signal, drives the electrooptical transducer 7 such asan LED with the driver 6 via the transmission controller 8 and is outputas the optical signal. In this case, the transmission controller 8 doesnot affect the normal operating state because the switching element 9selectively outputs the output of the driver 6.

In the testing state for the optical connection, the output of thedriver 6 does not reach the electrooptical transducer 7 such as an LEDbecause the switching element 9 outputs the analog signal input. In thiscase, the electrooptical transducer 7 directly receives the analogsignal which is input to the optical transmitter 2 because the analogsignal is directly input to the electrooptical transducer 7. Therefore,the current flowing through the electrooptical transducer 7 can becontrolled by controlling the input signal.

Next, the optical receiver 3 will now be described.

The optical receiver 3 includes an optoelectrical transducer 11, adecoder 12, an output buffer 13, and a receiving controller 14. Theseare formed in the same semiconductor substrate to provide a one-chipstructure.

The optoelectrical transducer 11 converts a received optical signal intoan electrical signal and outputs the electrical signal as a receivingsignal. The optoelectrical transducer 11 includes a light receivingelement 11 a such as photodetector (PD) that receives an optical signaland a transimpedance amplifier 11 b that converts a current output fromthe light receiving element 11 a into a voltage and outputs the voltage.

The decoder 12 consists of a clock regenerator 12 a that regenerates aclock signal from an output of the optoelectrical transducer 11, i.e.,the receiving signal, and a data decoder 12 b that regenerates theoutput of the analog-to-digital converter 4. In other words, the decoder12 outputs a regenerated digital signal and a regenerated clock signal.In the case where the analog-to-digital converter 4 of the opticaltransmitter 2 is a ΔΣADC, the regenerated digital signal is one-bitdigital signal and can be output as a regenerated analog signal by beingpassed through a low-pass filter. The clock regenerator 12 a consistsof, for example, a delay lock loop (DLL), a phase-locked loop (PLL),etc.

The output buffer 13 is a circuit that drives an output load and outputsthe regenerated digital signal V_(OUT) and the regenerated clock signalCLK. The input and the output of the decoder 12 and an input of theoutput buffer 13 are controlled by the receiving controller 14.

The receiving controller 14 outputs one selected from the output of thedecoder 12 and the output of the optoelectrical transducer 11 dependingon an average duty ratio of the output of the optoelectrical transducer11.

In other words, the receiving controller 14 outputs the output of theoptoelectrical transducer 11 to the output buffer 13 via the decoder 12when the average duty ratio of the output of the optoelectricaltransducer 11 is within decision values (thresholds). The receivingcontroller 14 outputs the output of the optoelectrical transducer 11 tothe output buffer 13 when the average duty ratio of the output of theoptoelectrical transducer 11 is not within decision values (thresholds).

Here, the average duty ratio of the output of the optoelectricaltransducer 11 is a time average of the output of the optoelectricaltransducer 11, i.e., the receiving signal, and the average duty ratio isobtained by, for example, passing the receiving signal through alow-pass filter. An averaging time can be set by a cut off frequency ofthe low-pass filter.

The decision values are the values compensated for the specified valuesof the average duty ratio D of the transmitting signal of the output ofthe encoder 5 with the amount of pulse-width distortion of theelectrooptical transducer 7 and optoelectrical transducer 11. Thethresholds are the upper limit and the lower limit of the decisionvalues. As described above, the specified values of the average dutyratio D of the transmitting signal of the output of the encoder 5 arevalues defined by the encoding scheme of the optical transmitter 2. Theamount of pulse-width distortion is a difference of H periods of thetransmitting signal and the output of the optoelectrical transducer 11,i.e., the receiving signal, and is the amount of change of duty ratios.

The receiving controller 14 includes switching elements 15 and 16 and aswitch controller 17.

The switching element 15 outputs the output of the optoelectricaltransducer 11 to the decoder 12 or the switching element 16. Theswitching element 16 outputs the output of the decoder 12 or the outputof the optoelectrical transducer 11 via the switching element 15. Theswitch controller 17 controls the switching elements 15 and 16.

FIG. 2 is a circuit diagram illustrating the configuration of a switchcontroller of the optically coupled insulating device illustrated inFIG. 1.

As illustrated in FIG. 2, the switch controller 17 includes a low-passfilter 20, inverters (INVs) 21 to 23, and a logical AND (AND) 24. Thelow-pass filter 20 outputs the average value of the output of theoptoelectrical transducer 11. The output voltage of the low-pass filter20 varies depending on the duty ratio of the output of theoptoelectrical transducer 11 and is middle value V_(DD)/2 when the dutyratio is 50%. Here, V_(DD) is a supply voltage. The low-pass filter 20may be an active filter or a passive filter including resistors andcapacitors.

The output of the low-pass filter 20 is input to the INVs 21 and 22. Thelogical threshold voltages of the INVs 21 and 22 are set to the boundaryvalues of the decision values of the duty ratio. For example, V_(DD)·¾and V_(DD)/4 are set, respectively. The output of the INV 22 is invertedby the INV 23 and is input to the AND 24. The AND 24 outputs the logicalmultiplication of the output of the INV 21 and the output of the INV 23.

The AND 24 outputs H when the average duty ratio is within the specifiedvalues and outputs L when the average duty ratio is not within thespecified values. In the case where the logical threshold voltages ofthe INVs 21 and 22 are set to, for example, V_(DD)·¾ and V_(DD)/4,respectively, the AND 24 outputs H when the output voltage of thelow-pass filter 20 is from V_(DD)/4 to V_(DD)·¾; and the AND 24 outputsL when the output voltage of the low-pass filter 20 is from 0 toV_(DD)/4 or from V_(DD)·¾ to V_(DD). Alternatively, the outputs of theswitch controller 17 may be inverted depending on the logic. The AND 24may output L when the output voltage of the low-pass filter 20 is fromV_(DD)/4 to V_(DD)·¾; and the AND 24 may output H when the outputvoltage of the low-pass filter 20 is from 0 to V_(DD)/4 or from V_(DD)·¾to V_(DD).

Referring again to FIG. 1, the switch controller 17 controls theswitching elements 15 and 16 depending on the average duty ratio of theoutput of the optoelectrical transducer 11. In other words, when theaverage duty ratio is within the decision values, the switch controller17 controls the switching elements 15 and 16 so that the output of theoptoelectrical transducer 11 is input to the decoder 12 and the outputof the decoder 12 is input to the output buffer 13. In contrast, whenthe average duty ratio is not within the decision values, the switchcontroller 17 controls the switching elements 15 and 16 so that theoutput of the optoelectrical transducer 11 is input to the output buffer13.

The output of the optoelectrical transducer 11 may be output to eitherthe regenerated digital signal V_(OUT) or the regenerated clock signalCLK when the average duty ratio is not within the decision values asdescribed above. Hereinafter, the case is described where the output ofthe optoelectrical transducer 11 is output to the regenerated clocksignal CLK.

The optically coupled insulating device 1 includes the opticaltransmitter 2 and the optical receiver 3. Each of the opticaltransmitter 2 and the optical receiver 3 is formed in a semiconductorsubstrate. These are packaged and provided as one device. The input andthe output of the optically coupled insulating device 1 are electricallyinsulated because a ground of the optical transmitter 2 and a ground ofthe optical receiver 3 are independent from each other.

The electrooptical transducer 7 of the optical transmitter 2 and theoptoelectrical transducer 11 of the optical receiver 3 are coupled withthe optical signal. In other words, the optical connection 18 consistsof the electrooptical transducer 7 and optoelectrical transducer 11including the light receiving element 11 a and the transimpedanceamplifier 11 b.

The light intensity of the light signal reaching to the light receivingelement 11 a of the optoelectrical transducer 11 varies due to variousfactors, for example, the temperature, aging degradation of the lightemitting element, and the like.

For example, in the case where the light emitting element of theelectrooptical transducer 7 is a LED and the light receiving element isa PD, the light intensity P_(O) received by the PD is shown in formula(1).

$\begin{matrix}{P_{O} = {\frac{CE}{100} \times \eta \times {If}}} & (1)\end{matrix}$

Here, If (A) is a current flowing through the LED, η (W/A) is luminousefficacy of the LED, and CE (%) is coupling efficiency of the LED andthe PD.

For example, the current If flowing through the LED is affected byvariations of IC processes. The luminous efficacy η degrades due to hightemperature and aging degradation. The coupling efficiency CE isaffected by variations of assembling, aging degradation of mold andresin, and the like. Therefore, the light intensity P_(O) received bythe light receiving element 11 a may vary with various values.

The light intensity P_(O) received by the light receiving element 11 aneeds to be greater than a certain value for error-free transmission tothe optical receiver 3. The conversion efficiency of the light receivingelement 11 a is taken as Se (A/W), and the input current for error-freetransmission to the optical receiver 3 is taken as I_(IN) (A). Then, arelation shown in formula (2) is required.

$\begin{matrix}{{I_{IN} \geq {{Se} \times P_{O}}} = {{Se} \times \frac{CE}{100} \times \eta \times {If}}} & (2)\end{matrix}$

The optical connection 18 needs to be tested in a testing processbecause error-free transmission is impossible when formula (2) is notsatisfied. The aim of testing for the optical connection 18 isverification that error-free transmission is possible under temperaturechange and aging degradation of a LED and the like. Herein, “error-freetransmission” denotes that an error rate of the receiving signal issufficiently low.

Simplified testing methods are preferable in the testing process. Assimplified methods, there is a method for testing whether data can betransmitted from the optical transmitter 2 to the optical receiver 3under simulated variation of the light intensity due to temperaturechange and aging degradation. In other words, the method is formeasuring the output of the optoelectrical transducer 11 while thecurrent of the electrooptical transducer 7 is forcibly and externallycontrolled. Namely, only products in which the output of theoptoelectrical transducer 11 is within specified values are shippedafter passing a current smaller than that in the normal operating state.Further, it is preferable that the testing process described above isexecuted by a direct-current.

As described above, the optical transmitter 2 of the optically coupledinsulating device 1 according to the embodiment is set in the normaloperating state or the testing state for the optical connectiondepending on the input level of the analog signal to be input. Forexample, when the input level V_(IN) of the analog signal input is lessthan V_(IN) _(—) _(MAX), the optically coupled insulating device 1 isset in the normal operating state. When the input level V_(IN) of theanalog signal input is greater than V_(IN) _(—) _(MAX), the opticallycoupled insulating device 1 is set in the testing state for the opticalconnection. Here, V_(IN) _(—) _(MAX) is the maximum value of the inputlevel of the analog-to-digital converter 4.

As described above, the average duty ratio D of the transmitting signalis within the specified values determined by the encoding scheme in thenormal operating state. The average duty ratio D of the transmittingsignal is 0 or 1 in the testing state for the optical connection becausethe analog signal to be input is a direct-current.

The optical receiver 3 is automatically set in the normal operatingstate or the testing state for the optical connection depending on theaverage duty ratio of the output of the optoelectrical transducer 11,i.e., the receiving signal. The optical receiver 3 is automatically setin the normal operating state or the testing state for the opticalconnection depending on whether the average duty ratio of the receivingsignal is within the decision values or not.

Therefore, the optical transmitter 2 is set in the testing state for theoptical connection and is able to output the analog signal in adirect-current by setting the input level of the optically coupledinsulating device 1 in the level of the testing state for the opticalconnection. At the time, the optical receiver 3 is automatically set tothe testing state for the optical connection because the average dutyratio of the output of the optoelectrical transducer 11 is not withinthe decision values. In other words, the output of the optoelectricaltransducer 11 is directly output.

As described above, in the testing state for the optical connection, theelectrooptical transducer 7 responds to the analog signal input to theoptically coupled insulating device 1, and the output of theoptoelectrical transducer 11 is directly output via the output buffer13. The output of the optically coupled insulating device 1 is adirect-current output with respect to the analog signal input.

Therefore, controlling the input current can control, the currentflowing through the electrooptical transducer 7. The output voltage ofthe optoelectrical transducer 11 at the time is detected by the outputvoltage of the optically coupled insulating device 1, and the voltagecan be tested whether it is within the specified values. This testingfor the optical connection is performed by a direct-current.

In the case where the input level of the optically coupled insulatingdevice 1 is the level of the normal operation, the optical transmitter 2is set in the normal operating state. In the normal operating state, theanalog signal input is converted into the digital signal; encoded withthe clock signal; and emitted from the electrooptical transducer 7 asthe optical signal.

In this case, the optical receiver 3 is controlled to the normaloperating state because the average duty ratio of the output of theoptoelectrical transducer 11 is within the decision values.

As described above, in the optical receiver 3 of the optically coupledinsulating device 1 according to the embodiment, control to the testingstate for the optical connection or the normal operating state isautomatically performed by the receiving controller 14 depending on theaverage duty ratio of the output of the optoelectrical transducer 11.

For example, in the case where the transmission controller 8 is notused, the current of the electrooptical transducer 7 cannot be forciblyand externally controlled. Therefore, it is not suitable for thedirect-current testing of the testing method described above. In otherwords, because the encoder 5 of the optical transmitter 2 outputsedged-pulse signals and PWM signals but cannot perform a direct-currentoperation, the testing described above cannot be performed in theproduct testing process.

In the case where the receiving controller 14 is not used, the opticalreceiver 3 firstly needs to detect whether the optical transmitter 2 isin the testing state for the optical connection or the normal operatingstate. As described above, the operating state of the opticaltransmitter 2 can be changed externally by the input level. However, itis generally difficult to change externally the operating state of theoptical receiver 3.

In the case where the optical receiver 3 is in the normal operatingstate, both the regenerated digital signal V_(OUT) and the regeneratedclock signal CLK output from the output buffer 13 are digital signals.Therefore, each potential of the signals varies from the minimumpotential to the maximum potential, i.e., the supply voltage V_(DD).

Therefore, to control externally the operating state of the opticalreceiver 3, the optical receiver 3 needs to be in the testing state forthe optical connection when a voltage of not less than the supplyvoltage V_(DD) and not more than a withstand voltage V_(MAX) is input tothe output buffer 13.

As described above, in the case where the operating state of the opticalreceiver 3 is controlled using an external signal controls, the supplyvoltage V_(DD), the withstand voltage V_(MAX), and a control voltageV_(CNT) must satisfy the relation of V_(DD)<V_(CNT)<V_(MAX). However, anultra-fine process cannot be used to satisfy this withstand voltageV_(MAX), and this results in increasing in cost of semiconductorcircuits. Therefore, to control externally the operating state of theoptical receiver 3 is generally difficult.

In contrast, the operating state of the optically coupled insulatingdevice 1 according to the embodiment can be automatically determined bythe received optical signal because the optical receiver 3 includes thereceiving controller 14.

Therefore, in the optically coupled insulating device 1 according to theembodiment, the optical connection 18, i.e., the optical connection ofthe electrooptical transducer 7 and the light receiving element 11 a canbe easily tested.

In addition, as described above, in the case where the ΔΣADC is used asthe analog-to-digital converter 4, the regenerated analog signal can beoutput by passing the digital signal V_(OUT) through a low-pass filter.

FIG. 3 is a block diagram illustrating the configuration of anotheroptical transmitter according to the embodiment.

As illustrated in FIG. 3, the optical transmitter 2A includes ananalog-to-digital converter 4A, an encoder 5, a driver 6, anelectrooptical transducer 7, and a transmission controller 8A. Theanalog-to-digital converter 4A, the encoder 5, the driver 6, and thetransmission controller 8A are formed in the same semiconductorsubstrate to provide a one-chip structure.

In other words, the optical transmitter 2A has a configuration in whichthe analog-to-digital converter 4 and the transmission controller 8 ofthe optical transmitter 2 illustrated in FIG. 1 are replaced with theanalog-to-digital converter 4A and the transmission controller 8A,respectively. The encoder 5, the driver 6, and the electroopticaltransducer 7 are similar to those of the optical transmitter 2illustrated in FIG. 1.

The analog-to-digital converter 4A differs from the analog-to-digitalconverter 4 illustrated in FIG. 1 in differential inputs having anon-inverting input and a inverting input.

The transmission controller 8A includes a switching element 9 and aswitch controller 10. The switching element 9 outputs an output of thedriver 6 or an inverting input of the analog signal. The switchcontroller 10 controls the switching element 9 depending on an inputlevel V_(IN+) of a non-inverting input of the analog signal.

For example, the switch controller 10 controls the switching element 9to the normal operating state when the non-inverting input level V_(IN+)of the analog signal is less than V_(IN) _(—) _(MAX). The switchcontroller 10 controls the switching element 9 to the testing state forthe optical connection when the non-inverting input level V_(IN+) of theanalog signal is greater than V_(IN) _(—) _(MAX). Here, V_(IN) _(—)_(MAX) is the maximum value (full scale) of the input level of theanalog-to-digital converter 4A.

The transmission controller 8A selectively outputs the output of thedriver 6 in the case of the normal operating state and the invertinginput of the analog signal in the case of the testing state for theoptical connection to the electrooptical transducer 7.

Therefore, in the normal operating state, the analog signal input isconverted into the digital signal by the analog-to-digital converter 4Aand encoded by the encoder 5 to the transmitting signal in which theoutput of the analog-to-digital converter 4A is superimposed onto theclock signal. Then, the output of the encoder 5, i.e., the transmittingsignal, drives the electrooptical transducer 7, for example, a LED, viathe transmission controller 8A by the driver 6, and is emitted as theoptical signal. In this case, the transmission controller 8A does notaffect the normal operation because the switching element 9 selectivelyoutputs the output of the driver 6.

In the testing state for the optical connection, the output of thedriver 6 does not reach the electrooptical transducer 7, for example, aLED, because the switching element 9 selectively outputs the invertinginput of the analog signal. In this case, a current drawn in theinverting input flows through the electrooptical transducer 7.Therefore, the current flowing through the electrooptical transducer 7can be controlled by controlling the current of the inverting input.

The current of the electrooptical transducer 7 in the testing state forthe optical connection is easily controlled because the opticaltransmitter 2A has differential inputs.

The optically coupled insulating device 1A is configurable by using theoptical transmitter 2A.

The optically coupled insulating device 1A includes an opticaltransmitter 2A and an optical receiver 3. Here, the optical receiver 3is similar to the optical receiver 3 illustrated in FIG. 1. Each of theoptical transmitter 2A and the optical receiver 3 is formed in asemiconductor substrate. These are packaged and provided as one device.The input and the output of the optically coupled insulating device 1Aare electrically insulated because a ground of the optical transmitter2A and a ground of the optical receiver 3 are independent from eachother.

The electrooptical transducer 7 of the optical transmitter 2A and theoptoelectrical transducer 11 of the optical receiver 3 are coupled atthe optical connection 18.

Each of the optical signals emitted from the electrooptical transducer 7in the testing state for the optical connection and the normal operationstate is similar to that of the optical transmitter 2.

The optical transmitter 2A is controlled to the testing state for theoptical connection by setting the non-inverting input level V_(IN+) ofthe analog signal to the level of the testing state for the opticalconnection, for example, V_(IN+)>V_(IN) _(—) _(MAX). The opticaltransmitter 2A is controlled to the normal operating state by settingthe non-inverting input level V_(IN+) of the analog signal to the levelof the normal operating state, for example, V_(IN+)<V_(IN) _(—) _(MAX).The optical receiver 3 is automatically set in the normal operatingstate or the testing state for the optical connection as describedabove.

In the testing state for the optical connection, the inverting input ofthe optically coupled insulating device 1A is connected to theelectrooptical transducer 7 and is directly output via theoptoelectrical transducer 11 of the optical receiver 3 to, for example,a regenerated clock terminal. In testing state for the opticalconnection, the output of the optically coupled insulating device 1A isa direct-current output with respect to the analog signal input.

Therefore, the current flowing through the electrooptical transducer 7is controlled by controlling the inverting input current. Measurementsof the inverting input current of the optically coupled insulatingdevice 1A and the output voltage of the optoelectrical transducer 11 atthe time enable testing whether the output voltage is within thespecified values. This testing process for the optical connection isperformed by a directly-current. For example, by passing a currentsmaller than that in the normal operating state through the LED, atesting can be performed whether a good optical connection is possibleeven in the case where the emission intensity of the LED is weak. Inother words, the product testing can be performed to verify theoperation under degradation of luminous efficacy due to temperatureraise and aging degradation.

In the normal operating state, the analog signal input is converted intothe digital signal; encoded with the clock signal; and emitted from theelectrooptical transducer 7 as the optical signal.

The optical receiver 3, which has received this optical signal, isautomatically set in the normal operating state and outputs theregenerated digital signal V_(OUT) and the regenerated clock signal CLK.As described above, the regenerated analog signal may be output bysmoothing the regenerated digital signal V_(OUT) with a low-pass filter.

In the optically coupled insulating device 1A according to theembodiment, the optical connection 18 can be easily tested because thenon-inverting input level V_(IN+) can change the operating state to thenormal operating state or the testing state for the optical connection.

The current flowing through the electrooptical transducer 7 may beeasily controlled because the optically coupled insulating device 1Aaccording to the embodiment has differential inputs.

As described above, the non-inverting input level V_(IN+) controls theswitch controller 10 of the transmission controller 8A. However, theinverting input level V_(IN−) may control the switch controller 10, andthe non-inverting input may be input to the electrooptical transducer 7via the switching element 9.

As described above, each of the optical receivers 3 and 3A is controlledto the normal operating state or the testing state for the opticalconnection depending on the average duty ratio of the output of theoptoelectrical transducer 11. Therefore, the encoding scheme in whichthe average duty ratio D of the transmitting signal is an appropriatevalue is preferable.

The output of the encoder 5, i.e., the average duty ratio D of thetransmitting signal varies depending on the digital signal output fromthe analog-to-digital converter 4 or 4A.

The case where a ΔΣADC that outputs, for example, one-bit data is usedas the analog-to-digital converter 4 or 4A will now be described. Thatis, the output digital signal of the ΔΣADC is one bit of H or L.

The case is described where the encoding scheme of the encoder 5 isconverted into the pulse of the duty ratio D₁ or D₂, respectively whenthe digital signal is H or L in the sampling period (sampling frequencyf_(S)). The amount of the pulse-width distortion of the opticalconnection 18 is taken as Δtp. In other words, the H period of thedigital signal input to the optical connection 18 varies by the amountof the pulse-width distortion Δtp at the output of the opticalconnection 18.

In the above condition, the average duty ratio D_(H) of the output ofthe optical connection 18 is shown in formula (3) when the digitalsignal is always H.D _(H) =D ₁ ±Δtp·f _(S)×100(%)  (3)

The average duty ratio D_(L) of the output of the optical connection 18is shown in formula (4) when the digital signal is always L.D _(L) =D ₂ ±Δtp·f _(S)×100(%)  (4)

Therefore, the average duty ratio of the output of the opticalconnection 18, i.e., the receiving signal, is within the values D_(L)and D_(H) given in formulas (3) and (4), respectively.

For example, in the case of encoding with the pluses of D₁=25% andD₂=75%, the average duty ratio is 25% to 75% even if the amount of thepulse-width distortion Δtp is zero. Further, if the clock frequency(sampling frequency) f_(S) is 10 MHz and the amount of the pulse-widthdistortion Δtp is ±10 ns, the average duty ratio is 15% to 85%.

The signal having this average duty ratio is passed through a low-passfilter to detect the average duty ratio of the output of the opticalconnection 18. In the case above, the output of the low-pass filter is0.15V_(DD) to 0.85V_(DD), and this fluctuates widely depending on theoutput pattern of the analog-to-digital converter 4 or 4A. Herein,V_(DD) is the supply voltage.

Therefore, the decision values of the average duty ratio of thereceiving controller 14 of the optical receiver 3 are 0.15V_(DD) to0.85V_(DD). The optical receiver 3 goes into the normal operating statewhen the average duty ratio is within the decision values, i.e.,0.15V_(DD) to 0.85V_(DD). The optical receiver 3 goes into the testingstate for the optical connection when the average duty ratio is notwithin the decision values, i.e., less than 0.15V_(DD) or greater than0.85V_(DD).

On the other hand, the receiving controller 14 preferably constructed asa digital circuit not as an analog circuit with respect to powerconsumption and a chip area. However, a digital circuit in which thelogical threshold is not within 0.15V_(DD) to 0.85V_(DD) may have alarge chip area and may be disadvantage in cost. Therefore, the encodingscheme in which the average duty ratio D is independent from the digitalsignal of the output of the analog-to-digital converter 4 or 4A ispreferable.

The encoding schemes in which the average duty ratio D does notfluctuate with respect to the data pattern of the digital signalinclude, for example, an encoding scheme using the Manchester code. Inthis encoding scheme, when the digital signal input in the samplingperiod is H, a signal HL is assigned such that H is in the first halfperiod of the sampling period and L is in the second half periodthereof. Further, when the digital signal is L, a signal LH is assignedsuch that L is in the first half period of the sampling period and H isin the second half period thereof. The signal LH is an inverted signalof the signal HL described above.

Each of the signals HL and LH described above has a duty ratio of 50%,and the transmission speed becomes double. However, the average dutyratio D is independent from the data pattern and is 50%. Here, thesampling period is 1/f_(S), where f_(S) is a clock frequency which isequal to the sampling frequency.

Therefore, the average duty ratio of the output of the opticalconnection 18 is 50±Δtp×f_(S)×100% when the Manchester code is used. Forexample, in the above numerical example, the average duty ratio is 40%to 60% and the switch controller 17 of the receiving controller 14 iseasily constructed as a digital circuit.

However, in the case where the decoder 12 regenerates the regeneratedclock signal from the output of the optical connection 18, i.e., thereceiving signal, the Manchester code is unsuitable when the raisingedge of the receiving signal is used.

FIG. 4 is a block diagram illustrating the configuration of stillanother optical transmitter according to the embodiment.

As illustrated in FIG. 4, the optical transmitter 2B includes ananalog-to-digital converter 4A, an encoder 5A, a driver 6, anelectrooptical transducer 7, a transmission controller 8A, and ascramble signal generator 19. In the optical transmitter 2B, theanalog-to-digital converter 4A, the encoder 5A, the driver 6, thetransmission controller 8A, and the scramble signal generator 19 areformed in the same semiconductor substrate to provide a one-chipstructure.

In other words, the optical transmitter 2B has a configuration in whichthe encoder 5 of the optical transmitter 2A illustrated in FIG. 3 isreplaced with the encoder 5A and the scramble signal generator 19 isadded. The analog-to-digital converter 4A, the driver 6, theelectrooptical transducer 7, and the transmission controller 8A aresimilar to those of the optical transmitter 2A illustrated in FIG. 3.

The scramble signal generator 19 generates a scramble signal, i.e.,outputs H or L synchronized with a sampling period. For example, thescramble signal generator 19 outputs an alternating signal synchronizedwith the raising of the clock signal or pseudo random binary bitsequence (PRBS) generated by the clock signal.

The encoder 5A generates the transmitting signal depending on two-bitdata of the output of the analog-to-digital converter 4A and the outputof the scramble signal generator 19.

The case where a ΔΣADC that outputs, for example, one-bit data is usedas the analog-to-digital converter 4A will now be described. That is,the output digital signal of the ΔΣADC is one bit of H or L.

The encoder 5A is a PWM encoder in which the two-bit data of thescramble signal and the output of the ΔΣADC are encoded into four pulseshaving duty ratios of W₁, W₂, W₃, and W₄ and the pulse periodsynchronized with the clock signal. The transmitting signal is thesignal in which the output of the ΔΣADC and the scramble signal aresuperimposed onto the signal based on the clock signal.

FIG. 5 is a truth table of inputs and outputs of the encoder 5A.Decoding can be also performed based on the truth table shown in FIG. 5.

Herein, the probability that the digital signal of the output of theΔΣADC is going to be H is taken as x, and the probability that thescramble signal is going to be H is taken as y. The average duty ratioD=D (x, y) of the output of the encoder 5A is shown in formula (5).

$\begin{matrix}{{D\left( {x,y} \right)} = {{{\left( {1 - x} \right)\left( {1 - y} \right)W_{1}} + {{x\left( {1 - y} \right)}W_{2}} + {\left( {1 - x} \right){yW}_{3}} + {xyW}_{4}} = {W_{1} + {x\left( {W_{2} - W_{1}} \right)} + {y\left( {W_{3} - W_{1}} \right)} + {{xy}\left( {W_{1} - W_{2} - W_{3} + W_{4}} \right)}}}} & (5)\end{matrix}$

As shown in formula (5), the average duty ratio D is dependent on theprobability x that the digital signal of the output of the ΔΣADC isgoing to H. The average duty ratio D is independent from the probabilityx when formula (6) is satisfied.

$\begin{matrix}{\frac{\partial{D\left( {x,y} \right)}}{\partial x} = {{\left( {W_{2} - W_{1}} \right) + {y\left( {W_{1} - W_{2} - W_{3} + W_{4}} \right)}} = 0}} & (6)\end{matrix}$

Therefore, the average duty ratio D is independent from the probabilityx when the combination of duty ratios W₁, W₂, W₃, and W₄ of the pulseperiod satisfies formula (6) with respect to the scramble signal of theprobability y.

Further, in the case where the probability y of the scramble signalvaried, the value of formula (7) needs to be minimized in order tominimize the influence on the encoder 5.

$\begin{matrix}{\frac{\partial{D\left( {x,y} \right)}}{\partial y} = {\left( {W_{3} - W_{1}} \right) + {x\left( {W_{1} - W_{2} - W_{3} + W_{4}} \right)}}} & (7)\end{matrix}$

Therefore, when the combination of duty ratios of W1, W2, W3, and W4 ofthe pulse period are satisfied formula (6) and further minimize thevalue of formula (7), the average duty ratio D of the output of theencoder 5A is in a constant value with respect to any value of theoutput of the ΔΣADC.

FIRST EXAMPLE

Here, an example of the duty ratio of the pulse period which satisfiesformula (6) will now be described.

For example, when each of the probability of H and the probability of Lof the scramble signal is 50%, i.e., y=0.5, and the duty ratios of thepulse period are W₁=20%, W₂=40%, W₃=80%, and W₄=60%, respectively,formula (6) is satisfied.

FIG. 6 schematically shows pulse waveforms and a truth table of inputsand outputs of the encoder 5A in the case of using the numerical examplerecited above.

Therefore, in the combinations, the average duty ratio D of the outputof the encoder 5A is in a constant value with respect to any value ofthe output of the ΔΣADC. The average duty ratio D=D (x, y) being in thisconstant value is obtained from formula (8), which is an alteration offormula (5).

$\begin{matrix}\begin{matrix}{{D\left( {x,y} \right)} = {W_{1} + {y\left( {W_{3} - W_{1}} \right)} + {x\left( \frac{\partial{D\left( {x,y} \right)}}{\partial x} \right)}}} \\{= {W_{1} + {y\left( {W_{3} - W_{1}} \right)}}}\end{matrix} & (8)\end{matrix}$

According to formula (8), the average duty ratio D of the output of theencoder 5A is determined by the probability y of H of the scramblesignal and the duty ratios W₁ and W₃ of the pulses. In the case of usingthe numerical example recited above, the average duty ratio D of theoutput of the encoder 5A is kept in a constant value of 50% because thevalue of the formula (8) is 50%. Therefore, the same effect can beobtained as the case of using the Manchester code.

In the case where the duty ratios of the pulses illustrated above areset to W₁=20%, W₂=40%, W₃=80%, and W₄=60%, the minimum pulse width is20% of the clock signal. For example, when the encoder 5A and thedecoder 12 are constructed with synchronous circuits, the operatingspeed is five times faster than the clock signal.

SECOND EXAMPLE

The encoder 5A may be constructed as the PWM encoder of which inputs areencoded into three pulses. For example, the duty ratios of W₁=W₃=0.5 andW₂+W₄=1 of pulses satisfy formula (6). Further, it can be set to, forexample, W₂=0.25 and W₄=0.75. Under these conditions, the L output ofthe ΔΣADC is encoded into the pulse having a duty ratio of 50%; and theH output of the ΔΣADC is encoded into the pulse having a duty ratio of25% or 75% depending on the L or H of the scramble signal, respectively.The duty ratios of W₂=W₄=0.5 and W₁+W₃=1 of pulses satisfy also formula(6). Further, it can be set to, for example, W₁=0.25 and W₃=0.75. Underthese conditions, the H output of the ΔΣADC is encoded into the pulsehaving a duty ratio of 50%; and the L output of the ΔΣADC is encodedinto the pulse having a duty ratio of 25% or 75% depending on the L or Hof the scramble signal, respectively.

FIG. 7 schematically shows pulse waveforms and a truth table of inputsand outputs of the encoder 5A in the case of using the numerical examplerecited above.

In the case where the duty ratios of the pulses are set to W₁=W₃=0.5,W₂=0.25, and W₄=0.75 and the case where the duty ratios of the pulsesare set to W₂=W₄=0.5, W₁=0.25, and W₃=0.75, the cases satisfy formula(6) as described above. Therefore, the average duty ratio D of theoutput of the encoder 5A is kept in a constant value of 50% with respectto any output value of the ΔΣADC. Further, under these conditions, theminimum pulse width is 25% in the period of the clock signal, and theoperating speed of the encoder 5A and the decoder 12 are decreased tofour times of the clock signal.

As described above, the scramble signal may be an alternating signalsynchronized with the raising of the clock signal or a PRBS generated bythe clock signal. For example, the scramble signal generator 19 mayconsist of a PRBS generator in which the output is inverted for everyclock signal. The PRBS generator includes a toggle flip-flop (TFF) or m(m being an integer not less than 2) flip-flops (FFs).

It is preferable to use the PRBS rather than the alternating signal asthe scramble signal because the output of the ΔΣADC may alternate H andL. However, when the ΔΣADC is equal to or greater than the second-order,this is not applicable.

The optically coupled insulating device 1B includes an opticaltransmitter 2B and an optical receiver 3. Here, the optical receiver 3is similar to the optical receiver 3 illustrated in FIG. 1. Each of theoptical transmitter 2B and the optical receiver 3 is formed in asemiconductor substrate. These are packaged and provided as one device.The input and the output of the optically coupled insulating device 1Bare electrically insulated because a ground of the optical transmitter2B and a ground of the optical receiver 3 are independent from eachother.

The electrooptical transducer 7 of the optical transmitter 2B and theoptoelectrical transducer 11 of the optical receiver 3 are coupled bythe optical signal. In other words, the input and the output of theoptically coupled insulating device 1B are coupled by the optical signalof the optical connection 18.

The optical signal emitted from the electrooptical transducer 7 issimilar to those of the optical transmitters 2 and 2A.

The optical transmitter 2B is controlled to the testing state for theoptical connection or the normal operating state depending on thenon-inverting input level V_(IN+) of the analog signal. The opticalreceiver 3 is automatically set to the normal operating state or thetesting state for the optical connection.

In the testing state for the optical connection, the inverting input ofthe optically coupled insulating device 1B is connected to theelectrooptical transducer 7 and is directly output via theoptoelectrical transducer 11 of the optical receiver 3. For example, theinverting input is output to the regenerated clock terminal. In testingstate for the optical connection, the output of the optically coupledinsulating device 1B is a direct-current with respect to the analogsignal input. This testing process for the optical connection isperformed by a directly-current.

In the normal operating state, the analog signal input is converted intothe digital signal, encoded with the clock signal, and emitted from theelectrooptical transducer 7 as the optical signal.

The optical receiver 3, which has received this optical signal, isautomatically set to the normal operating state and outputs theregenerated digital signal V_(OUT) and the regenerated clock signal CLK.As described above, the regenerated analog signal may be output bysmoothing the regenerated digital signal V_(OUT) with a low-pass filter.

In the optically coupled insulating device 1B according to theembodiment, the optical connection 18 can be easily tested because thenon-inverting input level V_(IN+) can change the operating state to thenormal operating state or the testing state for the optical connection.

In the optically coupled insulating device 1B, the average duty ratio Dof the output of the encoder 5A is kept in a constant value in thenormal operating state. Therefore, the decision values of the averageduty ratio of the optical receiver 3 can be set in the narrow range, andthe receiving controller 14 of the optical receiver 3 may be easilyconstructed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modification as would fall within the scope andspirit of the inventions.

The invention claimed is:
 1. An optical transmitter comprising: ananalog-to-digital converter configured to convert an analog signal intoa digital signal; an encoder configured to generate a transmittingsignal by superimposing an output of the analog-to-digital converteronto a signal based on a clock signal, the transmitting signal beingencoded to have an average duty ratio of more than zero and less thanone; a transmitting controller configured to output one of thetransmitting signal and the analog signal depending on an input level ofthe analog signal; and an electrooptical transducer configured toconvert an output of the transmitting controller into an optical signaland to output the optical signal.
 2. The transmitter according to claim1, further comprising a scramble signal generator configured to generatea scramble signal synchronized with the clock signal, theanalog-to-digital converter being configured to output one bit digitalsignal, and the encoder being configured to generate the transmittingsignal by encoding two-bit data of the output of the analog-to-digitalconverter and the scramble signal synchronized with the clock signal topulses, the pulses having duty ratios of a pulse period synchronizedwith the clock signal of W₁, W₂, W₃, and W₄, respectively, y, W₁, W₂,W₃, and W₄ being set to satisfy a formula ofW ₂ −W ₁ +y(W ₁ −W ₂ −W ₃ +W ₄)=0, where y is probability of a highlevel of the scramble signal.
 3. The transmitter according to claim 2,wherein the scramble signal is one of an alternating signal synchronizedwith the clock signal and a pseudo-random binary bit sequence signalgenerated by the clock signal.
 4. The transmitter according to claim 2,wherein y, W₁, W₂, W₃, and W₄ of the encoder are set to further satisfyformulas ofW₁=W₃=0.5,W ₂ +W ₄=1, andy=0.5.
 5. The transmitter according to claim 2, wherein y, W₁, W₂, W₃,and W₄ of the encoder are set to further satisfy formulas ofW₂=W₄=0.5,W ₁ +W ₃=1, andy=0.5.
 6. The transmitter according to claim 2, wherein y of the encoderis set to y=0.5 and W₁, W₂, W₃, and W₄ of the encoder are set to W₁=50%,W₂=25%, W₃=50%, and W₄=75% or to W₁=25%, W₂=50%, W₃=75%, and W₄=50%. 7.The transmitter according to claim 2, wherein y, W₁, W₂, W₃, and W₄ ofthe encoder are set to y=0.5, W₁=20%, W₂=40%, W₃=80%, and W₄=60%.
 8. Thetransmitter according to claim 2, wherein W₁, W₂, W₃, and W₄ of theencoder are set to further minimize a value of W₃−W₁+x(W₁−W₂−W₃+W₄),where x is probability of a high level of the output of theanalog-to-digital converter.
 9. An optical receiver comprising: anoptoelectrical transducer configured to receive one of an optical signalencoded to have an average duty ratio of more than zero and less thanone and an optical signal of an analog signal having an average dutyratio of zero or one and to convert the one of the optical signals intoan electrical signal; a decoder configured to decode an output of theoptoelectrical transducer and to output a regenerated digital signal anda regenerated clock signal; and a receiving controller configured tooutput one of an output of the decoder and the output of theoptoelectrical transducer depending on an average duty ratio of theoutput of the optoelectrical transducer.
 10. The receiver according toclaim 9, wherein the receiving controller includes a low-pass filterconnected to the optoelectrical transducer.
 11. An optically coupledinsulating device comprising: an optical transmitter; and an opticalreceiver, the optical transmitter including: an analog-to-digitalconverter configured to convert an analog signal into a digital signal;an encoder configured to generate a transmitting signal by superimposingan output of the analog-to-digital converter onto a signal based on aclock signal, the transmitting signal being encoded to have an averageduty ratio of more than zero and less than one; a transmittingcontroller configured to output one of the transmitting signal and theanalog signal depending on an input level of the analog signal; and anelectrooptical transducer configured to convert an output of thetransmitting controller into an optical signal, the optical receiverincluding: an optoelectrical transducer configured to receive theoptical signal and to convert the optical signal into an electricalsignal; a decoder configured to decode an output of the optoelectricaltransducer and to output a regenerated digital signal and a regeneratedclock signal; and a receiving controller configured to output one of anoutput of the decoder and the output of the optoelectrical transducerdepending on an average duty ratio of the output of the optoelectricaltransducer.
 12. The device according to claim 11, wherein the opticaltransmitter further includes a scramble signal generator configured togenerate a scramble signal synchronized with the clock signal, theanalog-to-digital converter is configured to output one bit digitalsignal, and the encoder is configured to generate the transmittingsignal by encoding two-bit data of the output of the analog-to-digitalconverter and the scramble signal synchronized with the clock signal topulses, the pulses having duty ratios of a pulse period synchronizedwith the clock signal of W₁, W₂, W₃, and W₄, respectively, y, W₁, W₂,W₃, and W₄ being set to satisfy a formula ofW ₂ −W ₁ +y(W ₁ −W ₂ −W ₃ +W ₄)=0 where y is probability of a high levelof the scramble signal.
 13. The device according to claim 12, whereinthe scramble signal is one of an alternating signal synchronized withthe clock signal and a pseudo-random binary bit sequence signalgenerated by the clock signal.
 14. The device according to claim 12,wherein y, W₁, W₂, W₃, and W₄ of the encoder are set to further satisfyformulas ofW₁=W₃=0.5,W ₂ +W ₄=1, andy=0.5.
 15. The device according to claim 14, wherein W₁, W₂, W₃, and W₄of the encoder are set to W₁=50%, W₂=25%, W₃=50%, and W₄=75%.
 16. Thedevice according to claim 12, wherein y, W₁, W₂, W₃, and W₄ of theencoder are set to further satisfy formulas ofW₂=W₄=0.5,W ₁ +W ₃=1, andy=0.5.
 17. The device according to claim 16, wherein W₁, W₂, W₃, and W₄of the encoder are set to W₁=25%, W₂=50%, W₃=75%, and W₄=50%.
 18. Thedevice according to claim 12, wherein y, W₁, W₂, W₃, and W₄ of theencoder are set to y=0.5, W₁=20%, W₂=40%, W₃=80%, and W₄=60%.
 19. Thedevice according to claim 12, wherein W₁, W₂, W₃, and W₄ of the encoderare set to further minimize a value of W₃−W₁+x(W₁−W₂−W₃+W₄), where x isprobability of a high level of the output of the analog-to-digitalconverter.
 20. The device according to claim 12, wherein the receivingcontroller includes a low-pass filter connected to the optoelectricaltransducer.